Device and method for cooling a planar inductor

ABSTRACT

The invention relates to a device for cooling a planar inductance coil, in particular a planar transformer, on a plate-shaped support having a plurality of conducting layers, wherein at least one conducting layer of the support, in co-operation with a core element designed to guide a magnetic flux, provides the planar inductance coil, wherein on its first side towards a surface of the support the core element is connected to said side by means of a heat-conducting adhesive and on a second planar outside surface it is preferably substantially over the entire surface glued to a cooling element having a planar contact surface.

The present invention concerns a device and a method for cooling aplanar inductance coil, in particular a planar transformer, on aplate-shaped support having a plurality of conducting layers, wherein atleast one conducting layer of the support, in co-operation with a coreelement designed to guide a magnetic flux, represents the planarinductance coil.

A typical area of use of devices of that kind, of the general kind setforth, are switching power supplies. In this context, due to increasingminiaturization, multi-layer support plates (referred to as“multi-layer” members) are increasingly used, which have a plurality ofconducting layers within a conventional circuit board structure, theconducting layers being electrically separated from each other or beingconnected in point configuration. In this area of use for example alsoconventional discrete inductance coils such as for example transformersor chokes are being afforded by use of the planar technology, morespecifically by directly utilizing suitably designed conducting layersof the multi-layer member as windings of that inductance coil, in whichcase they then usually co-operate with a transformers core which issuitably placed on the multi-layer member or in openings therethrough.

The use of such planar inductance coils of the general kind set forth ishowever made difficult in particular in regard to power electronics by anumber of mechanical and thermal problems. Thus more specifically forexample in switching power supplies in a very small space copper andcore losses are incurred, which without particular cooling measurescause an excessive rise in temperature of the multi-layer conductorsupports so that even for example when over-dimensioning is involved theuse of this novel technology encounters power limits.

Particularly in the case of devices with a relatively high level of(lost) power therefore attempts have been made to additionally cool themulti-layer member by various measures, with for example so-called“thermal drains”, that is to say heat sinks, in the form of metal pinsor the like, being used in relation to a cooling body. An arrangement ofthat kind, which is to be found in the state of the art, is illustratedfor the sake of simplicity in FIG. 4 of the accompanying drawing: atransformer arrangement or choke in a multi-layer member 10 withconducting layers which are designed accordingly as transformer windingshas a first transformer core 12 which for example is of an E-shapedconfiguration in cross-section and which extends with limbs 14 throughcorresponding openings in slot form in the multi-layer member 10. Toclose the magnetic circuit, disposed on the first transformer core 12 isa second plate-shaped transformer core 16 which is of an I-shapedconfiguration in cross-section so that winding layers which extend forexample in the interposed multi-layer portions 18 are embraced by thetransformer core 12, 16. The core elements 12, 16 are glued together inlateral relationship or in surface relationship and thus guarantee themagnetic circuit.

To cool this arrangement—which as stated is known from the state of theart—shown in the left-hand region of FIG. 4 is a spacer pin 20 which ispressed into the board or plate 10 and which at the other end affordsthermal contact with a plate-shaped cooling body 22. An alternativewhich is also known from the state of the art is shown in the right-handregion of FIG. 4; in that case, a cooling pin 24 is soldered directlyinto the board 10 and—like also the spacer pin 20—connected to thecooling body 22 by means of a screw connection.

Such an arrangement however gives rise to a series of damaging safetyand thermal expansion problems and furthermore space is additionallyrequired on the circuit board 10 due to the thermal transfer and spacerportions 20, 24 respectively. The rigid connection involved is alsounsatisfactory and liable to trouble, in particular in relation toacceleration phenomena or in the event of a severe mechanical loading. Afurther disadvantage is that the heat is only dissipated in punctiformfashion by the thermal drains and furthermore the through holes whichare required for that purpose reduce the usable surface area of themulti-layer member even for the internally disposed layers.

A further approach which is to be found in the state of the art isillustrated in FIG. 5, showing thermal bonding of the transformer coreitself to the cooling body 22. That is effected by means of an elasticlayer 26 of heat-conducting material which is disposed between thetransformer core 16 and the cooling body 22 in the manner shown in FIG.5. The mechanical connection between the cooling body 22 and themulti-layer member 10 is afforded by way of spacer portions 28 andscrews 30; the dimensional tolerances which naturally occur in respectof the cores and bolts however necessitate flexibility on the part ofthe material 26 which, in the form of a flexible heat-conducting mat oflarge area, is also referred to as a “gap pad” or “soft pad”. Besidesheat dissipation to the cooling body still being unsatisfactory, due tothe transfer conditions involved, the arrangement shown in FIG. 5therefore also gives rise to not inconsiderable production andmanufacturing expenditure. The FIG. 5 arrangement also suffers from thesame disadvantages as the construction shown in FIG. 4.

Finally, FIG. 6 shows a further approach to be found in the state of theart, in which heat of the multi-layer member 10 is discharged to thecooling body 22 by means of elastic heat-conducting mats 32; at the sametime the transformer arrangement can be held by a resilient clip element34. This arrangement however does not involve any cooling of the core.

All those arrangements however give rise to a not inconsiderable levelof expenditure and in addition are in particular not suitable for thedissipation of relatively large amounts of heat, governed by the powerinvolved. Furthermore this state of the art does not provide for anyfixing of the core; if necessary such fixing would have to beimplemented separately.

The severity of that problem is increased when planar transformers areused in a so-called matrix arrangement; a plurality of transformerswhich are arranged in a distributed array on a multi-layer member andwhich each require individual local heat dissipation.

Finally, there would in principle also be the possibility of sealing atransformer arrangement on a multi-layer member with a heat-conductingcasting material in order in addition to cool the arrangement. The poortestability and reparability of this arrangement however is evidenthere, as well as the basically rather poor suitability of castingmaterials for dissipating heat; in addition cores and further componentsare subjected to mechanical loadings.

Therefore the object of the present invention, for multi-layer supportsof the general kind set forth, with fitted planar inductors, is toprovide a heat dissipation means which is in particular even suitablefor high levels of power loss and which is mechanically stable and whichin addition permits simple, inexpensive and potentially automatableproduction.

That object is attained by the apparatus set forth in claim 1 and theuse as set forth in claim 9; advantageous developments of the inventionare set forth in the appendant claims. Advantageously the inventionmakes it possible to provide a planar inductance coil in a multi-layermember, in particular a circuit arrangement in power electronics, whichis extremely simple in terms of manufacture, which is suitable forautomatic fitment or implementation and which in addition permits a veryhigh degree of heat dissipation—both from the heat-generating portion ofthe multi-layer member and also from the transformer core.

In accordance with the invention it has been found that the direct andimmediate connection of the cooling element which has a planar contactsurface to the core element allows arrangements with a high level ofpower loss, with correspondingly high heat generation, without the fearof for example damage to the arrangement. In accordance with theinvention the transformer cores are viewed not just as magnetic orelectrical components but as mechanical elements which—by virtue oftheir relatively good thermal conduction, for example in the case offerrite—serve as heat bridges and fix the multi-layer structuralassembly. The cores, with the shortest spacing, also provide the largestpossible surface area for the dissipation of heat at the location atwhich it occurs.

This approach is significant in particular in relation to multi-layermembers having a plurality of distributed cores in which acorrespondingly large number of independent cores have to be cooled, asboth the mechanical expenditure and complication is reduced in relationto the constructions from the state of the art, which involve expensiveadditional parts, while in addition the dissipation of heat can be mademore efficient. Large-area cooling is thus made possible withoutinvolving additional mechanical components, with the heat beingdissipated directly at the location at which it is generated (that is tosay the transformer winding or core).

In addition the adhesive layer according to the invention canadvantageously compensate for tolerance problems between the variouscores of a matrix arrangement and the plate-shaped cooling element. Inparticular then the thickness of the multi-layer circuit board and thethickness of the cores no longer play any part in terms of mechanicalfixing.

In addition the core elements which are made from brittle material, forexample ferrite, are advantageously reliably fixed, whereby the assemblyis extremely vibration-resistant.

Particularly when a continuous metal cooling plate of large surface areais advantageously used as the cooling element, this suitably serves as ascreening means in relation to interference fields of the inductors.

It is also in accordance with the invention, for the connectionsaccording to the invention, to use electrically conductive adhesiveswhich, as they are electrically conductive, often also possess goodthermal conductivity; in regard to heat dissipation therefore, there areconsiderable advantages in comparison with insulating plastic materialsas are used for example for casting and sealing purposes.

In addition it has been found to be advantageous to use the coolingelement according to the invention in addition for coolingsemiconductors or other heat-generating electronic components on thesupport board (multi-layer member), so as to afford a complete, compactand efficient cooling and assembly system for electrical power modules.

In accordance with a development moreover it is particularly preferablypossible for the cooling element according to the invention to be sopositioned relative to the electronic components to be cooled that bothcooling of the core element and of the electronic component which isadditionally to be cooled can be effected within a single workingoperation or assembly operation; this can be suitably effected forexample by suitably dimensioned projections or profiled portions of thecooling element at engagement and contact locations for a powersemiconductor to be cooled. As a result that affords a cooling system inparticular also for SMD-equipped arrangements, without incurringadditional expense.

Finally a further advantage of the arrangement according to theinvention is that the—expensive—multi-layer surface is kept free fromadditional mechanical fixing elements, and instead room is afforded forfurther peripheral electronics, for example for SMD-equipment, and/oradditional safety spacings.

Further advantages, features and details of the invention will beapparent from the following description of preferred embodiments andfrom the accompanying drawings in which:

FIG. 1 is a diagrammatic plan view of a circuit board arrangement to becooled in accordance with the invention, with a plurality ofdistributedly arranged transformers and chokes,

FIG. 2 is a side view in section through a planar inductance coil to becooled in accordance with a first preferred embodiment of the invention,

FIG. 3 shows a side view in section of a further embodiment of theinvention with additional semiconductor power elements, and

FIGS. 4 through 6 show procedures for cooling planar inductance coilsfrom the state of the art.

For the purposes of describing the embodiments of FIGS. 1 through 3,reference numerals corresponding to FIGS. 4 through 6 are employed ifthey involve identical components.

FIG. 1 shows a plan view of a power semiconductor arrangement with amulti-layer circuit board 10 and a plate-shaped, planar cooling body 22of ordinary cooling body material, for example copper or aluminum.

Arranged on the circuit board 10 is a plurality of transformers (orchokes) 38—in part distributed in matrix form—, wherein thosetransformers (cores and windings) are held and cooled on their sideremote from the fitment or components side shown in FIG. 1, by contactwith the cooling body 22 involving an entire surface area.

In addition FIG. 1 shows a plurality of (SMD-fitted) electroniccomponents 40 on the fitment or components side of the board 10, and itis also possible to see a plurality of power semiconductor elements 42which are also cooled by contact with the cooling body 22.

FIG. 2 now shows as a diagrammatic side view the basic principle of theinvention. In the manner already described hereinbefore, the firsttransformer core 12, and the second transformer core 16, enclosingportions 18 of the board 10, are in the form of planar transformers. Inaccordance with the invention in addition the E-shaped first transformerelement 12 is connected by means of a for example electricallyconducting, heat-conductive adhesive connection 44 to the downwardlydirected surface of the multi-layer member 10 between the limbs 14, andthe flat surface of the transformer core 12 is connected over its entirearea by means of an electrically conductive and heat-conductive adhesive46 to the cooling body plate 22. The adhesive used for the adhesiveconnections 44 and 46 respectively preferably has metal particles or thelike which not only afford electrical conductivity between thecomponents involved, but in addition also provide for markedly superiorthermal conductivity. In relation to the magnetic properties of thecores which are cooled in that way however the electrical connectionbetween the transformer core and the cooling body is practically withoutdisadvantageous consequences.

FIG. 3 illustrates the arrangement in principle in accordance with theinvention as shown in FIG. 2 in the environment of a heat-generatingpower module such as for example an electronic switching power supply.Disposed adjacent the transformer arrangement 12, 6 is a powersemiconductor 42, for example an insulated switching transistor, whichis also connected to the cooling body 22 in the illustrated manner byway of an adhesive connection 48 and which thus not only makes use ofthe existing cooling surface area but in addition also provides forfurther mechanical stabilization of the arrangement. A correspondingconsideration applies for the portion-wise, direct, heat-dissipatingcontacting of the multi-layer in the region of the projection 50 of thecooling body 22, as well as lateral fixing and cooling of the powertransistor 42′ which is connected by way of an intermediate layer(insulation) 52 to a suitably formed portion of the cooling body 22.

In the illustrated fashion, it is possible to provide for thermally andmechanically optimized thermal dissipation for power multi-layer memberswith integrated transformers or chokes.

In addition it is possible for the illustrated arrangements to beproduced by a substantially automated production apparatus which ideallyalso in conjunction with SMD-fitment/soldering permits the production ofa complete power module to be automated. Particularly when dealing withrelatively large numbers of items, it is possible in that way to providefor inexpensive production, combined with reproducible coolingproperties.

As a supplemental aspect, the invention permits the additional coolingof SMD-power components, for example in casings such as D-pack, D²-pack,SOT 223 and so forth, without additional expenditure. The lost heatproduced is dissipated to the external cooler through the multi-layermember; this can be seen for example in FIG. 3 above the projection 50.In addition, for improving thermal conduction, copper or the likethermally conducting material can advantageously be introduced into themulti-layer member, beneath the power components, wherein the layers canbe connected together with vias.

In addition the adhesive generally adapts to any unevenness so that notonly is the thermal contact or transfer resistance due to enclosed airbetween all components involved reduced; in addition, the adhesiveaffords an effective surface-equalization effect. After the adhesivesets, the parts in addition can no longer be displaced relative to eachother; this not only affords a reliable, durable, thermal connection butalso a vibration-resistant, mechanical connection which can suitablycarry loadings.

For further optimization of the invention, the different coefficients ofexpansion of the multi-layer member and the cooling plate can preferablybe adapted to each other. As a power multi-layer member of that kindcontains a very great deal of copper, the thermal linear expansion ofsuch a plate is approximately equal to that of copper (multilayer memberFR 4: 10-17 10⁻⁶/K; copper: 16.5 10⁻⁶/K; ferrite: 10.5 10⁻⁶/K).

With a typical adhesive thickness of about 150 micrometers, it isrelatively small and affords a correspondingly low level ofheat-transfer resistance. Besides adhesives in particular which can beapplied in fluid form, a double-sided, thermally conducting adhesivefoil or sheet is also possible, for one or each of the two adhesiveconnections.

What is claimed is:
 1. A device for cooling (a) a planar inductance coil located on a plate-shaped support (10) having a plurality of conducting layers, wherein at least one conducting layer of the support represents the planar inductance coil, and (b) a core element (12, 16) designed to guide magnetic flux, characterized in that said core element includes a first side and a second planar outside surface; on said first side, which is towards a surface of the support (10), the core element is connected to said support by means of a conducting adhesive (44), and said core element is glued on said second planar outside surface over substantially its entire second planar outside surface area to a cooling element (22) having a planar contact surface, thereby providing a thermal connection between said planar inductance coil, said coil and said cooling element, wherein said cooling element serves as a heat bridge for cooling both said core element and said planar inductance coil and for fixing the multi-layer structure assembly.
 2. A device as set forth in claim 1 characterized in that the cooling element is provided for additionally cooling a power semiconductor or the like heat-generating electronic component, which is disposed on the support (10).
 3. A device as set forth in claim 2 characterised in that in a contact region (50) with the power semiconductor (42) the cooling element has a projection or a suitably profiled portion.
 4. A device as set forth in claim 1 characterized in that provided on the plate-shaped support is a plurality of planar inductance coils which are preferably arranged at regular spacings and which each have a respective core element, a common cooling element being glued to the core elements.
 5. A device as set forth in claim 1 characterized in that the cooling element is of a plate-shaped configuration and is adapted to extend substantially parallel to the support (10).
 6. A device as set forth in claim 5 characterised in that the cooling element extends substantially over an entire surface of the plate-shaped support (10).
 7. A device as set forth in claim 1 characterized in that gluing between the core element and the support and/or gluing between the core element and the cooling element is effected with an adhesive in a thickness of between 100 and 200 micrometers.
 8. A device as set forth in claim 1 characterized in that gluing between the core element and the support and/or between the core element and the cooling element is effected by means of a double-sided, thermally conducting adhesive foil.
 9. A device for cooling (a) a planar inductance coil located on a plate-shaped support (10) having a plurality of conducting layers, wherein at least one conducting layer of the support represents the planar inductance coil, and (b) a core element (12, 16) designed to guide magnetic flux, characterized in that said core element includes a first side and a second planar outside surface; on said first side, which is towards a surface of the support (10), the core element is connected to said support by means of a heat-conductive conducting adhesive (44), and said core element is glued on said second planar outside surface over substantially its entire second planar outside surface area to a cooling element (22) having a planar contact surface, thereby providing a thermal connection between said planar inductance coil, said coil and said cooling element, wherein said cooling element, serving as a heat bridge and for fixing the multi-layer structure assembly, is provided for additionally cooling a power semiconductor or the like heat-generating electronic component which is disposed on the support (10) and, wherein in a contact region (50) the cooling element has a projection or a suitably profile portion. 